Free-Electron Lasers are devices, where the kinetic energy of relativistic electrons is transferred to a coherent radiation beam. Because the radiation wavelength can be tuned by the energy of the driving electron beam, the FEL does not suffer from wavelength limitation as for the case of quantum lasers. This allows the exploration of a coherent radiation source in the VUV and X-ray regime.
The key element of an FEL is the undulator, a magnet with an alternating polarity and a given period length. Electrons, injected into the undulator, will emit spontaneous radiation (undulator radiation). The radiation will eventually act back on the electron beam, resulting in a periodic modulation in the energy and current, as well as the coherent emission at this periodicity.
PBPL operated the first successful SASE FEL in the IR wavelength with a gain in the FEL output power of more than 5 orders of magnitude. Since then the group has become an active member in the FEL field, both experimentally and analytical. It currently participates in the multi-institutional FEL experiments VISA and LCLS, as well as continues the research in the IR region with its own FEL at PEGASUS.
An extensive list of all Free-Electron Lasers around the world can be found at the "Virtual Library: Free Electron Lasers" at the University of California, Santa Barbara